When a direct current potential is applied across an emulsion, suspension or solution containing charged particles or molecules, the latter migrate toward the electrode bearing the opposite charge. This phenomenon is generally called elctrophoresis and is utilized to apply coatings onto metallic surfaces for purposes of electrical insulation, paint priming, weather protection, and the like.
Most of the polymer electrophoretic deposition techniques followed in industry involve aqueous systems. However, aqueous depositions are in general markedly affected and vitiated by the evolution of gases at the electrodes. This gas evolution, deriving from water electrolysis can result in heavily pitted polymer coatings, which makes them particularly unsuitable for electrical insulation. Moreover, water emulsion systems are generally plagued by difficult-to-control surface tension, necessary pH conditions, and viscosity difficulties.
Polyimide resins have recently come into use as high temperature electrical insulating films. Polyimide films are generally produced by film casting of a non-aqueous solvent solution followed by a heat cure. Briefly, this involves dissolving a suitable polyamic acid polymer in a solvent, casting the solution uniformly upon a smooth surface and then slowly heating until a suitable polyimide insulating film is produced from the acid polymer derivative. A major problem with the solution casting method is that only relatively thin films (0.0001 inch for foil coatings) can be produced in a single coat. Heavier coating or film thickness required a multicoat system with each coat being dried and cured before applying the subsequent coat. Difficulty is also encountered with uniform coating of corners and particularly of sharp edges. Uniform polyimide coverage of irregular-shaped objects has proved practically impossible by solution casting and expensive equipment costs are encountered in this method.
As a partial solution, electrophoretic deposition techniques have been developed for polyamic acids in a water emulsion system, as described in U.S. Pat. No. 3,537,970. Such an aqueous polymer electrodeposition system, however still suffers the aforedescribed disadvantages of film pitting. Although most of the polymer electrophoretic deposition techniques applied in industry involve aqueous processes, a few organic systems, such as those shown in U.S. Pat. Nos. 3,450,655 and 3,463,714, have also been used. These systems have involved vinyl resins, epoxy resins, and carboxyl-containing polymers and copolymers such as polyacrylic acid, vinyl acetate/maleic acid copolymers, ethylene/itaconic acid copolymers and ethylene/maleic acid copolymers among others.
It has been found that a great number of variables exist in non-aqueous electro-deposited systems as regards the ratio between polymer and solvent and between solvents within the solvent system, and that each polymer used presents its own characteristic problems in its dilution or suspension and deposition.